Method and apparatus for modular redundancy with alternative mode of operation

ABSTRACT

A method and apparatus to provide triple modular redundancy (TMR) in one mode of operation, while providing multiple context selection during a second mode of operation. Intelligent voting circuitry facilitates both modes of operation, while further enhancing the robustness of the design when used in a TMR mode of operation. Various addressing schemes are provided, which allow dual use of the configuration data lines as selection signals using one addressing scheme, while allowing for dual use of the configuration address lines as selection signals using the second addressing scheme.

FIELD OF THE INVENTION

The present invention generally relates to modular redundancy, e.g.,triple modular redundancy (TMR), and more particularly to modularredundancy circuits and methods selectively exhibiting high reliabilityoperation or an alternative functionality, e.g., multiple contextoperation in a programmable logic device.

BACKGROUND

PLDs are a well-known type of integrated circuit that may be programmedto perform specified logic functions. One type of PLD, the FieldProgrammable Gate Array (FPGA), typically includes an array ofprogrammable tiles. These programmable tiles can include, for example,Input/Output Blocks (IOBs), Configurable Logic Blocks (CLBs), dedicatedRandom Access Memory Blocks (BRAM), multipliers, Digital SignalProcessing blocks (DSPs), processors, clock managers, Delay Lock Loops(DLLs), Multi-Gigabit Transceivers (MGTs) and so forth.

Each programmable tile typically includes both programmable interconnectand programmable logic. The programmable interconnect typically includesa large number of interconnect lines of varying lengths interconnectedby Programmable Interconnect Points (PIPs). The programmable logicimplements the logic of a user design using programmable elements thatmay include, for example, function generators, registers, arithmeticlogic, and so forth.

The programmable interconnect and the programmable logic are typicallyprogrammed by loading a stream of configuration data into internalconfiguration memory cells that define how the programmable elements areconfigured. The internal configuration memory cells control configurablepoints, such as CLB functionality or PIPs. The configuration data may beread from memory (e.g., from an external PROM) or written into the FPGAby an external device. The collective states of the individual memorycells then determine the function of the FPGA.

Another type of PLD is the Complex Programmable Logic Device, or CPLD. ACPLD includes two or more “function blocks” connected together and toInput/Output (I/O) resources by an interconnect switch matrix. Eachfunction block of the CPLD includes a two-level AND/OR structure similarto those used in Programmable Logic Arrays (PLAs) and Programmable ArrayLogic (PAL) devices. In some CPLDs, configuration data is stored on-chipin non-volatile memory. In other CPLDs, configuration data is storedoff-chip in non-volatile memory, then downloaded to volatile memory aspart of an initial configuration sequence.

For all of these PLDs, the functionality of the device is controlled bydata bits provided to the device for that purpose. The data bits can bestored in volatile memory (e.g., static memory cells, as in FPGAs andsome CPLDs), in non-volatile memory (e.g., FLASH memory, as in someCPLDs), or in any other type of memory cell.

Some PLDs, such as the Xilinx Virtex® FPGA, can be programmed toincorporate blocks with pre-designed functionalities, i.e., “cores”. Acore can include a predetermined set of configuration bits that programthe FPGA to perform one or more functions. Alternatively, a core caninclude source code or schematics that describe the logic andconnectivity of a design. Typical cores can provide, but are not limitedto, DSP functions, memories, storage elements, and math functions. Somecores include an optimally floor planned layout targeted to a specificfamily of FPGAs. Cores can also be parameterizable, i.e., allowing theuser to enter parameters to activate or change certain corefunctionality.

Programmable logic devices can be susceptible to functional failureunder certain circumstances. The memory cells, for example, that areused to program the PLD's functionality can inadvertently “flip”, or inother words, change their logic state. Such failures may be calledsingle event upsets (SEUs), or radiation induced errors, and can lead tofunctional failure of the design implemented by the FPGA.

SUMMARY

To overcome limitations in the prior art, and to overcome otherlimitations that will become apparent upon reading and understanding thepresent specification, the various embodiments of the present inventiondisclose an apparatus and method for a circuit that supports triplemodular redundancy (TMR) as well as a second mode of operation providingother functionality, e.g., multiple context operation in programmablelogic devices (PLDs).

In accordance with one embodiment of the invention, an integratedcircuit includes two or more signal sources (e.g., configuration memorycells in a PLD), two or more selection circuits (e.g., multiplexers),and a voting circuit. Each selection circuit has two or more inputterminals, with each input terminal being coupled to one of the signalsources. Each selection circuit also includes a mode select inputterminal and an output terminal, with each selection circuit beingadapted to provide a selected one of the input data signals to theoutput terminal. Each input terminal of the voting circuit is coupled tothe output terminal of one of the selection circuits. The voting circuitprovides to an output terminal a value that appears at a majority of theinput terminals. Each of the signal sources is provided to more thanhalf of the selection circuits. Thus, by applying certain values of themode select signals, the input data signals from each of the signalsources can be provided to more than half of the input terminals of thevoting circuit. The input data signal thus provided appears at theoutput terminal of the voting circuit. By selecting other values of themode select signals, the voting circuit functions as a modularredundancy circuit, e.g., a triple modular redundancy (TMR) circuit.

In some embodiments, the integrated circuit is a PLD, and the signalsources are configuration memory cells of the PLD. When the votingcircuit is a 3-input voting circuit, for example, the circuit functionsin a first mode to provide TMR protection for a configuration stored inall three of the configuration memory cells, and in a second mode toprovide a selected one of the three stored values from the configurationmemory cells.

In accordance with another embodiment of the invention, a method ofoperating an integrated circuit includes selecting one of first andsecond modes of operation for the IC, and providing a modular redundancyfunction in the first mode and an identity function in the second mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and advantages of the invention will become apparentupon review of the following detailed description and upon reference tothe drawings in which:

FIG. 1 illustrates an integrated circuit (IC) that exemplifies a FieldProgrammable Gate Array (FPGA) architecture;

FIG. 2 illustrates an exemplary configuration memory block diagram inaccordance with one embodiment of the present invention;

FIG. 3 illustrates a voting control circuit in accordance with oneembodiment of the present invention;

FIG. 4 illustrates an alternative voting control circuit in accordancewith another embodiment of the present invention;

FIG. 5 illustrates a configuration memory in accordance with oneembodiment of the present invention;

FIG. 6 illustrates an alternative configuration memory in accordancewith another embodiment of the present invention; and

FIG. 7 illustrates an exemplary memory cell in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION

Generally, the various embodiments of the present invention provide amethod and apparatus to provide triple modular redundancy (TMR) in onemode of operation, while providing multiple context selection during asecond mode of operation. Intelligent voting circuitry facilitates bothmodes of operation, while enhancing the robustness of the design whenused in a TMR mode of operation. Various addressing schemes areprovided, which allow dual use of the configuration data lines asconfiguration selection signals with a first addressing scheme, whileallowing for dual use of the configuration address lines asconfiguration selection signals using with a second addressing scheme.

As noted above, advanced integrated circuits (ICs), such as FPGAs, caninclude several different types of programmable logic blocks in thearray. For example, FIG. 1 illustrates an IC that exemplifies FPGAarchitecture 100, including a large number of different programmabletiles such as Multi-Gigabit Transceivers (MGTs) 101, CLBs 102, BRAMs103, IOBs 104, configuration and clocking logic CONFIG/CLOCKS 105, DSPs106, specialized I/O 107, including configuration ports and clock ports,and other programmable logic 108, such as digital clock managers,analog-to-digital converters, system monitoring logic, and so forth.Some FPGAs also include dedicated processor blocks PROC 110, in whichspecific CPU related functionality may be utilized that is separate fromthe FPGA fabric.

In some FPGAs, each programmable tile includes programmable interconnectelement INT 111 having standardized connections to and from acorresponding interconnect element in each adjacent tile. Therefore, theprogrammable interconnect elements taken together implement theprogrammable interconnect structure for the illustrated FPGA. INT 111also includes the connections to and from the programmable logic elementwithin the same tile, as shown by the examples of blocks 102 and 104.

For example, a CLB 102 may include a Configurable Logic Element CLE 112that may be programmed to implement user logic plus a singleprogrammable interconnect element INT 111. A BRAM 103 can include a BRAMlogic element (BRL 113) in addition to one or more programmableinterconnect elements. Typically, the number of interconnect elementsincluded in a tile depends on the height of the tile. In the picturedembodiment, a BRAM tile has the same height as four CLBs, but othernumbers (e.g., five) can also be used. A DSP tile 106 can include a DSPlogic element (DSPL 114) in addition to an appropriate number ofprogrammable interconnect elements. An IOB 104 may include, for example,two instances of an input/output logic element IOL 115 in addition toone instance of the programmable interconnect element INT 111.

As will be clear to those of skill in the art, the actual I/O padsconnected, for example, to the I/O logic element 115 are manufacturedusing metal layers above the various illustrated logic blocks, andtypically are not confined to the area of the input/output logic element115.

In the pictured embodiment, a columnar area near the center of the die(shown shaded in FIG. 1) is used for configuration, clock, and othercontrol logic. Horizontal areas 109 extending from this column are usedto distribute the clocks and configuration signals across the breadth ofthe FPGA.

Some FPGAs utilizing the architecture illustrated in FIG. 1 includeadditional logic blocks that disrupt the regular columnar structuremaking up a large part of the FPGA. The additional logic blocks can beprogrammable blocks and/or dedicated logic. For example, the processorblock PROC 110 shown in FIG. 1 may span several columns of CLBs andBRAMs.

Note that FIG. 1 is intended to illustrate only an exemplary FPGAarchitecture. The number of logic blocks in a column, the relative widthof the columns, the number and order of columns, the type of logicblocks included in the columns, the relative size of the logic blocks,and the interconnect/logic implementations 102, 103, and 104 are purelyexemplary. For example, in an actual FPGA more than one adjacent columnof CLBs is typically included wherever the CLBs appear, to facilitatethe efficient implementation of user logic.

As discussed above, configuration of a PLD may be performed via staticlatch memory cells that store control data, where each memory cellstores a single bit of control data. The control data may be used tocontrol the conductivity state of pass transistors in multiplexers, toserve as logic values in lookup tables, or to perform some otherconfiguration function. The control data bits can be stored in volatilememory (e.g., static memory cells, as in FPGAs and some CPLDs), innon-volatile memory (e.g., FLASH memory, as in some CPLDs), or in anyother type of memory cell. In various embodiments of the invention, thecontrol data bits may be organized into various configuration memoryblocks and appropriately accessed to support both a TMR and a multiplecontext mode of operation.

Turning to FIG. 2, configuration block diagram 200 in accordance withone embodiment of the present invention is exemplified. Configurationmemory blocks 206-210 represent a portion of configuration memory blocksthat may be present within a PLD, such as FPGA 100 of FIG. 1, or a CPLD,to control logic and routing of a configurable function within the PLD.Configuration memory blocks 206-210 may co-exist within the sameconfiguration memory array, or may exist in separate configurationmemory arrays.

Address bus 202 and data bus 204 combine to provide configuration memorydata frames to configuration memory blocks 206-210. For each addressapplied to configuration memory blocks 206-210 by address bus 202, databits from data bus 204, i.e., one data frame, may be written into thecorresponding memory cells of configuration memory blocks 206-210.

In accordance with one embodiment of the present invention, each dataframe written into configuration memory blocks 206-210 are identical.Thus, once configuration memory blocks 206-210 have been programmed,each configuration memory block 206-210 contains contents that areidentical. It should be noted, that while three configuration memoryblocks are illustrated, other quantities of memory blocks, such as 5, 7,9, etc. may be provided. As discussed in more detail below, the sameconfiguration data is written into an odd number of configuration memoryblocks, when implementation of a high reliability mode of operation isdesired.

In an alternative embodiment, configuration memory blocks 206-210 maycontain different data. In such an instance, each of configurationmemories 206-210 may contain configuration memory that defines acompletely different logic and routing configuration, i.e., context, ofthe PLD. Thus, through selection of configuration memory 206, forexample, the PLD may be configured as an operational component tosupport recording within a video recording device, whereas ifconfiguration memory 208 is selected, the PLD may be configured tooperate as an operational component to support video playback within thesame video device.

Through operation of control block 212 and multiplexer block 214, eithera high-reliability mode of operation, or a multiple context mode ofoperation is provided. In the high-reliability mode of operation,configuration memory blocks 206-210 are written with identical contents.Voting circuitry distributed between control block 212 and multiplexerblock 214 ensures that correct configuration data is provided forcorrect logic/routing definitions by selecting configuration data thatconforms to a majority rule.

That is to say, for example, that should any of the memory celllocations within either of configuration memory blocks 206-210 becontaminated, e.g., through operation of a single event upset, thenvoting circuitry within control block 212 and multiplexer block 214detects the disparity between the memory cells' contents. In particular,the voting circuitry compares the contaminated memory cell's contentswith the redundant memory cells' contents, and selects the correct logicvalue in accordance with the majority rule. Thus, if the logic valuesfor three memory cells are, for example, “1”, “1”, “0”, then the votingcircuitry selects a “1” to be the correct state of the configurationbit, which is then used to control the configurable point in the logicor routing. In general, given that an odd number of memory cells areanalyzed by the voting circuitry, a majority condition should alwaysexist to provide a valid majority rule comparison.

If, on the other hand, the contents of configuration memory blocks206-210 are different, then a multiple context mode of operation isdesired. In this instance, control block 212 and multiplexer block 214simply interoperate to provide the correct configuration, e.g., one ofconfiguration memory A, B, C, etc. to implement the desired logic androuting for the configurable function of the PLD.

It can be seen, therefore, that while one embodiment of the presentinvention provides the ability for a high-reliability mode of operation,an alternative mode of operation, i.e., multiple context configurationability, is also provided in another embodiment of the presentinvention. In such an instance, should the user of the PLD decide not touse the high reliability mode, he/she may instead choose to utilize theconfiguration memory blocks for alternate configurations. Configurationsof the PLD may be altered within a single clock cycle as will bediscussed in more detail below.

Turning to FIG. 3, a voter/multiplexer control circuit in accordancewith one embodiment of the present invention is exemplified. In thisembodiment, a three memory cell implementation is illustrated, in whichmemory cells 302-306 represent individual memory cells of correspondingfirst, second, and third configuration memory blocks as shown, forexample, by configuration memory blocks 206-210 of FIG. 2.

In particular, memory cell 302 may exist within configuration memoryblock 206, memory cell 304 may exist in configuration memory block 208,and memory cell 306 may exist in configuration memory block 210. Asdiscussed above, memory cells 302-306 may represent 3 bits of individualcontexts, 302, 304, and 306, for a multiple context mode of operation.Conversely, memory cells 302-306 may represent 3 bits initially havingidentical values, such as is the case in a high-reliability, e.g., TMRmode of operation.

While in a multiple context mode of operation, mode select 312 iseffective to select one of inputs A-C, which corresponds to memory cells302-306, respectively, via multiplexer 310. As such, the logic value ofthe selected memory cell is optionally latched by latch 314 andultimately provided to FPGA control point 316. FPGA control point 316 isthen operative to effect logic and/or routing within the FPGA inaccordance with the function that is associated with the logic value ofFPGA control point 316.

Alternatively, while in a high reliability mode of operation, modeselect 312 selects input 318 to be supplied to optional latch 314. Inthis instance, voting control circuit 308 receives the logic values ofmemory cells 302-306 and determines which logic value represents themajority of logic values contained within memory cells 302-306. Thetruth table of the operation of voting control circuit 308 isillustrated in Table 1. The Boolean function of equation (1) asimplemented by voting control circuit 308 is, for example,D=A&B|B&C|C&A,  (1)where D is the logic value of voting control circuit output 318, A isthe logic value contained within memory cell 302, B is the logic valuecontained within memory cell 304, C is the logic value contained withinmemory cell 306, “&” is the logical AND operator, and “|” is the logicalOR operator.

TABLE 1 CELL A CELL B CELL C VOTER OUTPUT 0 0 0 0 0 0 1 0 0 1 0 0 0 1 11 1 0 0 0 1 0 1 1 1 1 0 1 1 1 1 1Thus, given a majority number of logic low valued memory cells, i.e., 2or more out of 3, an output logic value of “0” will be selected byvoting control circuit 308 as output 318. On the other hand, given amajority of logic high valued memory cells, i.e., 2 or more out of 3, anoutput logic value of “1” will be selected by voting control circuit 308as output 318.

It should be noted that latch 314 is an optional component of FIG. 3. Inparticular, removing latch 314 would provide one less component in thesignal path between multiplexer 310 and FPGA control point 316. Removinglatch 314 may be advantageous, therefore, during a high-reliability modeof operation, since by removing latch 314, one less component exists inthe signal path that may contribute to a single event upset.

On the other hand, providing latch 314 in the signal path betweenmultiplexer 310 and FPGA control point 316 facilitates a multiplecontext mode of operation. In particular, while FPGA control point 316is at a logic level equivalent to the logic level of memory cell 302,for example, mode select 312 may flip the selection state of multiplexer310 to select the logic level of either memory cell B 304 or C 306,depending upon the context change that is desired. Once the logic valueof memory cell 304 or 306 is provided to the input of latch 314, modeselect 312 may then activate latch 314 to provide the logic value ofmemory cell 304 or 306 to FPGA control point 316. In this way,transition from one context to another may occur synchronously throughoperation of latch 314.

It should be noted, that while FIG. 3 illustrates activation of onememory cell from a selection of three memory cells, a plurality ofmemory cells may be similarly selected. When one of memory cells302-306, for example, are selected, many hundreds to many tens ofthousands of other memory cells may be similarly selected atcorresponding FPGA control points to implement a particular design. Insuch an instance, therefore, inputs A-C of multiplexer 310 may eachrepresent a different logic/routing configuration for three separatedesigns.

Through operation of latch 314, the FPGA is then able to toggle its modeof operation at every rising or falling edge of its latch signalreceived from mode select 312. That is to say, for example, that whilethe FPGA may be operating in accordance with a user design representedby the “A” memory cells, the user may select a design represented by the“B” memory cells, such that the logic level of the “B” memory cells ismade available at the input of latch 314. Upon activation of the triggersignal from mode select 312, the new design represented by the “B”memory cells is then activated within the FPGA and the FPGA functions inaccordance with the “B” design.

Turning to FIG. 4, an alternative voter/multiplexer control circuit isexemplified in accordance with another embodiment of the presentinvention. It can be seen by inspection, that 4-to-1 multiplexer 310 asillustrated by FIG. 3 is removed. Thus, instead of requiring amultiplexer to select which of memory cells A 402, B 404, or C 406 is tobe activated, the selection circuit of FIG. 4 instead biases the inputto voting control circuit 408 to force the desired selection.

Control signals SELECT A, SELECT B, and SELECT C are each mutuallyexclusive logic controls that have been previously decoded into logichigh, i.e., “True”, or logic low, i.e., “False”, logic levels. Inaddition, the inputs to 2-to-1 multiplexers 416-420 are cross-connectedin an appropriate fashion, such that when combined with the SELECT A,SELECT B, and SELECT C control signals, their respective output logiclevels are effective to create the required bias condition.

In operation, voting control circuit 408 implements equation (1) asdiscussed above when determining which memory cell is to be activated atFPGA control point 414. Control signals SELECT A, SELECT B, and SELECT Care each used to toggle the selection decision performed by multiplexers416-420. Appropriately selected, the control signals are effective toduplicate a particular input to voting control circuit 408 so as to biasits voting decision.

For example, when control signal, SELECT A, is at a logic high level,each of control signals SELECT B and SELECT C are at a logic low level.Thus, multiplexer 416 selects its first input, memory cell A 402, as itsoutput, multiplexer 418 selects its second input, memory cell A 402 asits output, and multiplexer 420 selects its first input, memory cell C406 as its output. The three inputs to voting control circuit 408 are,therefore, “A”, “A”, “C”, thus requiring that the logic level of memorycell A 402 be placed at output 410 by operation of the majority rule ofequation (1).

Similarly, when control signal, SELECT B, is at a logic high level, eachof control signals SELECT A and SELECT C are at a logic low level. Thus,multiplexer 416 selects its first input, memory cell A 402, as itsoutput, multiplexer 418 selects its first input, memory cell B 404 asits output, and multiplexer 420 selects its second input, memory cell B404 as its output. The three inputs to voting control circuit 408 are,therefore, “A”, “B”, “B”, thus requiring that the logic level of memorycell B 404 be placed at output 410 by operation of the majority rule ofequation (1).

Similarly, when control signal, SELECT C, is at a logic high level, eachof control signals SELECT A and SELECT B are at a logic low level. Thus,multiplexer 416 selects its second input, memory cell C 406, as itsoutput, multiplexer 418 selects its first input, memory cell B 404 asits output, and multiplexer 420 selects its first input, memory cell C406 as its output. The three inputs to voting control circuit 408 are,therefore, “C”, “B”, “C”, thus requiring that the logic level of memorycell C 406 be placed at output 410 by operation of the majority rule ofequation (1).

Thus, by asserting one of control signals SELECT A, SELECT B, or SELECTC, a multiple context mode of operation may be established. Through theuse of latch 412 and associated control input (not shown), a synchronouscontext change in the FPGA may be effected. In such an instance, thecorrect context may first be selected through operation of multiplexers416-420 and voting control circuit 408 as discussed above. Next, oncethe correct context is selected, it may then be latched into FPGAcontrol point 414.

It should be noted, that if optional latch 412 is removed, the entiresignal path from voting control circuit 408 and FPGA control point 414is completely void of programmable logic. Thus, by removing latch 412,signal path 410 may made to be less susceptible to single event upset.

In an alternative embodiment, a glitchless transition from one contextto another is achieved through appropriate control of select signalsSELECT A, SELECT B, or SELECT C. A glitchless transition guarantees thatif A and B are the same value, that the output does not temporarily takeon some different value. In particular, a two-step transition process isprovided, which guarantees that all three inputs to voting controlcircuit 408 are not the same.

For example, given that the current context is “A” and a contexttransition to “B” is desired, then the two-step transition illustratedin Table 2 is effective to ensure a glitchless transition from context“A” to “B”.

TABLE 2 OPERATION INPUT #1 INPUT #2 INPUT #3 VOTER OUTPUT select A A A CA select A and B A A B A deselect A and A B B B select B

In particular, the process begins with the “A” context having beenselected, where as discussed above, inputs #1, #2, and #3 into votingcontrol circuit 408 are “A”, “A”, and “C”, respectively, throughappropriate selection of signals SELECT A, SELECT B, and SELECT C. Next,both SELECT A and SELECT B are asserted to a logic high value and signalSELECT C remains deasserted at a logic low value. As such, the firstinput of multiplexer 416, memory cell A 402, and the second inputs ofmultiplexers 418 and 420, memory cells A and B, are applied to the inputof voting control circuit 408 to provide the logic value of memory cellA at output 410 as shown in Table 2.

Next, SELECT A is deasserted to a logic low value and SELECT B remainsasserted to a logic high value. As such, the first input of multiplexer416, memory cell A 402, the first input of multiplexer 418, memory cellB 404, and the second input of multiplexer 420, memory cell B, areapplied to the input of voting control circuit 408 to provide the logicvalue of memory cell B at output 410 as shown in Table 2. Thus, it canbe seen that a glitchless transition between contexts “A” and “B” isfacilitated.

A voter control circuit as illustrated, for example, in FIG. 4 isapplied to each set of three configuration memory cells for either ofTMR, or multiple context, modes of operation. It can be seen, therefore,that a substantial number of select lines is required to facilitateconfiguration changes for either mode.

Note that the circuit of FIG. 4 (e.g., including elements 408, 416, 418,and 420) can be used in systems and circuits other than multiple contextPLDs. For example, this circuit can be used in any application where itis desired to select between two modes of operation, TMR and an identityfunction. In a first mode, the circuit would apply TMR to the threeinput data signals (A, B, and C) from the three signal sources (e.g.,memory cells 402, 404, and 406). In a second mode, the circuit wouldselect one of the input data signals from one of the three signalsources, and apply the selected input data signal to two of the threeinput terminals of voting circuit 408. Therefore, because the votingcircuit implements a majority vote of the input signals, the selectedone of the input data signals would appear at the output terminal of thevoting circuit. The second mode could be used, for example, as a testmode in which each of the three input data signals is provided in turnto the output terminal of the voting circuit.

In one embodiment according to the present invention, each configurationframe is increased in size by a factor of three bits to provide for theadditional configuration memory cells as exemplified in FIG. 5. Inparticular, address lines are illustrated to run vertically and bitlines are illustrated to run horizontally, where in the illustratedembodiment, a total of 5 frames are shown. It should be noted, however,that virtually any number of frames may be supported by the variousembodiments of the present invention and the discussions herein arepresented merely for instructional purposes only.

In one embodiment, the organization of data frames is chosen so thatmultiple bits in a single frame store A, B and C values for a singleprogramming point. Bit 502, for example, represents the “A” copy of the0^(th) configuration bit. Similarly, bits 504 and 506 represent, forexample, the “B” and the “C” copies of the 0^(th) configuration bits.Thus, bits 502-506 may be used, for example, to initially program thelogic values of memory cells 302-306 as illustrated in FIG. 3, or memorycells 402-406 as illustrated in FIG. 4. The three bits below bit 506represent another programming point, for example, the 1^(st)configuration bits of the configuration frame. Similarly, data bits 508may represent the A, B, and C copies of the 0^(th) configuration bits ofthe next configuration frame, and so on. Thus, data bit group 510represents the first 5-bits of data for the “A” configuration, data bitgroup 512 represents the first 5-bits of data for the “B” configuration,and data bit group 514 represents the first 5-bits of data for the “C”configuration.

After all memory cells have been configured by the configuration data asexemplified in FIG. 5, then the data lines, e.g., 502-506, used toprogram the memory cells are available for other use. That is to say,that if the address lines of FIG. 5 are not activated for configuration,then the data lines are unused and are free to be used in anothercapacity. Thus, the data lines may also be used as the configurationselection lines, SELECT A, SELECT B, and SELECT C as illustrated in FIG.4.

After configuration, therefore, data line 502 selects “A” for the 0^(th)configuration bit, SELECT A₀, data line 504 selects “B” for the 0^(th)configuration bit, SELECT B₀, and data line 506 selects “C” for the0^(th) configuration bit, SELECT C₀. In such a way, the need to runseparate select lines for each selection circuit is obviated through thedual use of data lines as provided by one embodiment of the presentinvention.

In an alternative embodiment, the multiple bits corresponding to thesame configuration point are stored in identical bit locations insequential frames. The configuration controller may addressconfiguration frames sequentially according to a multi-frame writecapability as exemplified in FIG. 6. Multi-frame write capability isespecially convenient during a TMR mode configuration, since the samedata is being written for each configuration. In this instance, datalines 602, 610, 612, 614, and 616 represent, for example, data bitsA₀-A₄ of the first word of the “A” configuration that is addressed byaddress, X, for example. The address may then be incremented to X+1 andthe same data word may be written again starting with data line 604,which represents data bit, B₀, of the first word of the “B”configuration. The other data bits, B₁-B₄, are offset from data bit B₀in the same fashion as data bits A₁-A₄ are offset from A₀. The addressmay again be incremented to X+2 and the same data word may again bewritten to the first word of the “C” configuration.

Thus, it can be seen that the data word only needs to be transmittedonce and then can be subsequently written into 3 continuous addresslocations, X, X+1, and X+2, before a new data word is to be transmitted.Bits 608, for example, may represent the 1 st bit of the first word foreach configuration, i.e., A₁, B₁, and C₁, which only needed to betransmitted one time in order to update three separate memory cells.

In an alternative embodiment, multiple context configurations are alsosupported by the configuration diagram of FIG. 6. In particular, theconfiguration controller may write each data frame of a particularconfiguration before writing the data frames of another configuration.In such an instance, the data frames of configuration “A” are written indiscontinuous fashion by incrementing the write addresses using integeroffsets, e.g., X, X+3, X+6, etc. Similarly, the data frames ofconfiguration “B” are written in discontinuous fashion by incrementingthe write addresses using integer offsets, e.g., X+1, X+4, X+7, etc.Thus, all frames of a particular configuration, “A”, “B”, or “C”, may beconfigured before moving on to all of the frames of anotherconfiguration.

In order to utilize the addressing scheme of FIG. 6, the alternativememory cell configuration of FIG. 7 may be utilized, where multiplexer712 corresponds to multiplexer 416 of FIG. 4 and memory cell C 710corresponds to memory cell C 406 of FIG. 4. Cross-coupled inverters 708represent memory cell A 402 of FIG. 4. Two pass gates exist for thealternative memory cell of FIG. 7, where pass gate 706 is used to enablea global configuration state of the FPGA.

In operation, signal CONFIGURATION ENABLE is used to transition passgate 706 into a conductive state during an FPGA configuration process.At the same time, signal ADDRESS may render pass gate 704 conductive, ifmemory cell 708 is being programmed. If so, then both pass gates 704 and706 are conductive during configuration of memory cell 708, such thatsignal DATA BIT Ai is passed into latch 708 for storage.

Once the configuration process is complete, signal CONFIGURATION ENABLEis deasserted, which renders pass gate 706 non-conductive. In thisinstance, signal ADDRESS/SELECT Ci doubles as the selection control formultiplexer 712. If signal ADDRESS/SELECT Ci is at a logic low level,for example, then the logic value of memory cell A (from latch 708) isselected for the output of multiplexer 712. If, on the other hand,signal ADDRESS/SELECT Ci is at a logic high level, then the logic valueof memory cell C 710 is selected for the output of multiplexer 712.

It should be noted, that the alternative memory cell configuration ofFIG. 7 as discussed in relation to memory cell A 402 of FIG. 4 is alsoused to implement memory cells B 404 and C 406 as well. As such, thememory cell of FIG. 7 is instantiated for each memory cell that is usedin a multiple context mode of operation. Thus, in one embodiment, threememory cells as exemplified in FIG. 4 constitutes an implementationwhere a triple context mode of operation is desired.

Other aspects and embodiments of the present invention will be apparentto those skilled in the art from consideration of the specification andpractice of the invention disclosed herein. It is intended that thespecification and illustrated embodiments be considered as examplesonly, with a true scope and spirit of the invention being indicated bythe following claims.

1. An integrated circuit (IC), comprising: a plurality of signal sources providing input data signals; a plurality of selection circuits, each selection circuit having a plurality of input terminals each coupled to one of the signal sources, a mode select input terminal, and an output terminal, each selection circuit being adapted to provide a selected one of at least two of the input data signals to the output terminal of the selection circuit; and a voting circuit having a plurality of input terminals each coupled to the output terminal of one of the selection circuits and further having an output terminal, the voting circuit providing at the output terminal of the voting circuit a value appearing at a majority of the input terminals of the voting circuit, wherein each of the signal sources is provided to more than half of the selection circuits.
 2. The IC of claim 1, wherein: the plurality of signal sources comprises three signal sources; the plurality of selection circuits comprises three selection circuits, each having two input terminals; and a first set of values on the node select input terminals of the selection circuits enables a triple modular redundancy function of the input data signals, and a second set of values on the node select input terminals of the selection circuits enables an alternative function.
 3. The IC of claim 2, wherein the alternative function is an identity function of one of the input data signals.
 4. The IC of claim 2, wherein the alternative function is an identity function of a selected one of the input data signals, the selected one of the input data signals being selected from among the input data signals based on the second set of values.
 5. The IC of claim 1, wherein each selection circuit comprises a multiplexer circuit having a plurality of data input terminals each coupled to one of the signal sources, a select terminal coupled to the mode select input terminal of the selection circuit, and an output terminal coupled to the output terminal of the selection circuit.
 6. The IC of claim 1, wherein the IC comprises a programmable logic device (PLD), and the plurality of signal sources comprise memory cells of the PLD.
 7. The IC of claim 6, further comprising a programmable control point, wherein the output terminal of the voting circuit is coupled to provide a logic value received from the voting circuit to the programmable control point.
 8. A method of operating an integrated circuit (IC), comprising: selecting one of first and second modes of operation for the IC; providing a modular redundancy function of a plurality of input data signals during the first mode of operation; and providing a selected one of the input data signals to more than half of a plurality of input terminals of a voting circuit during the second mode of operation.
 9. The method of claim 8, wherein the IC comprises a programmable logic device (PLD), the method further comprising: configuring the PLD to store values for the input data signals in memory cells of the PLD.
 10. The method of claim 9, wherein the values for the input data signals comprise portions of three different configurations of the PLD, and selecting one of the first and second modes of operation for the IC comprises selecting the second mode of operation.
 11. The method of claim 8, wherein both the providing a modular redundancy function and the providing a selected one of the input data signals comprise selecting a value based on a majority vote of at least some of the input data signals.
 12. An integrated circuit (IC), comprising: means for selecting one of first and second modes of operation for the IC; and means for providing a modular redundancy function of a plurality of input data signals during the first mode of operation and providing a selected one of the input data signals during the second mode of operations wherein in the second mode of operation the selected one of the input data signals is provided to more than half of a plurality of input terminals of a voting circuit.
 13. The IC of claim 12, wherein the IC comprises a programmable logic device (PLD), the IC further comprising: means for storing values for the input data signals in memory cells of the PLD.
 14. The IC of claim 13, wherein the values for the input data signals comprise portions of three different configurations of the PLD, and the IC is operated in the second mode of operation.
 15. The IC of claim 12, wherein the means for providing a modular redundancy function of a plurality of input data signals during the first mode of operation and providing a selected one of the input data signals during the second mode of operation comprises means for selecting a value based on a majority vote of at least some of the input data signals. 